1. Field of the Invention
This process and apparatus relate to the conversion of uranium hexafluoride (UF.sub.6) to ammonium diuranate (ADU), with the ultimate purpose of conversion of the ADU by calcination to UO.sub.2 powder which can be pressed and sintered into fuel pellets for nuclear reactors.
2. Description of the Prior Art
U.S. Pat. No. 2,466,118 discloses that UF.sub.6 can be dissolved in water to form uranyl fluoride solutions from which ADU can be precipitated by either a single stage or two stage treatment with ammonium hydroxide, the two stage process with an intermediate digestion stage at from 55.degree. to 75.degree.C being emphasized as necessary to produce an easily separable precipitate.
U.S. Pat. No. 3,272,602 teaches as critical the precipitation of ADU from an aqueous solution of uranyl salt having a concentration of 100 grams/liter or less, adding ammonia of 28% concentration or less at a rate of 200 grams of NH.sub.3 per kilogram of U per minute or less, with a precipitation end point pH of from 6 to 7. At pH values of about 6, the ADU is an aggregate material, and the patentees state that this last crystalline product is provided by leading some of the ADU in solution. The specific examples are directed to a batch process, and the end point pH is 6.3 and 6.5 in these examples. No teaching relevant to a continuous process is disclosed.
DeHollander U.S. Pat. No. 3,394,997 discloses as necessary features for producing non-slimy ADU precipitates that dilute ammonia, substantially less than 2 molar, and preferably not over about 1.2, be added to uranyl fluoride solutions to produce an easily separable precipitate. Prior art use of concentrated ammonia solutions in treating UF.sub.6 hydrolysis produce was alleged to produce slimy precipitates that were quite difficult to dewater and dry. The process of this patent involves the use of surge or holding tanks to retain the ADU precipitate suspension in water for up to 11/2 hours to allow crystal growth to take place in order to produce a more easily filterable ADU. Large quantities of water are employed, some 5590 pounds being required for 130.4 pounds of UF.sub.6 in the detailed showing.
U.S. Pat. No. 3,579,311 refers to the DeHollander patent application that resulted in U.S. Pat. No. 3,394,997 and adopts the process steps thereof for producing ADU.
In U.S. Pat. No. 3,758,664 there is disclosed a four step process for producing ADU, namely (1) a UF.sub.6 -water hydrolysis mixing step to produce UO.sub.2 F.sub.2 ; (2) an initial partial neutralization of the UF.sub.6 hydrolysis product with concentrated ammonium hydroxide; (3) a subsequent addition to this initial product of more concentrated ammonium hydroxide (3 to 6 molar), for instance by spraying, to a pH of 9.5 or higher, including recirculating part of the ADU suspension in water by means of a pump, and (4) digesting the ADU slurry in a holding tank for a period of up to 30 minutes, followed by dewatering the suspension to produce a flowable ADU slurry.
Welty U.S. Pat. No. 3,726,750 teaches the conversion of UF.sub.6 to ADU by initially admixing a water hydrolysis solution of UF.sub.6 with dilute ammonium hydroxide (1.1 to 1.25 N) to a pH of 5.5 to 6, then more dilute ammonium hydroxide (about 1 normal) is admixed as by spraying to precipitate the uranium compounds in solution as ADU, and the resulting slurry is recirculated by means of a pump. Large volumes of water are employed-- over 3100 pounds for 100 pounds of UF.sub.6.
As some of the above listed prior art has indicated, there are some factors, not fully identified, that result in relatively wide variations in the ADU products which affect the ceramic properties of the UO.sub.2 produced thereform upon calcining. Sometimes an ADU product will calcine into a UO.sub.2 powder that when pressed into compacts and sintered under standard conditions produces fuel pellets all of uniform high density, for example 95 .+-. 2% of theoretical, of high strength and free from cracks, porosities or other defect, while the ADU product at other times calcined and treated identically to the former, produces fuel pellets whose densities vary widely and non-uniformly, and comprise substandard, cracked, porous and defective pellets. The problems of such prior art processes have been these uncontrollable variations in properties. The ultimate test of the properties of the ADU and the ceramic quality of the UO.sub.2 produced therefrom is the making of fuel pellets under standard conditions of pressing, sintering and so on, and checking the properties of the pellets.
It has been found that certain of the UO.sub.2 powder properties do correlate closely to the quality of pellets produced therefrom, as well as enabling prediction of the uniformity between pellets in a batch. The properties are:
1. Bult density -- B.rho. (Scott density in grams/cc.), PA1 2. Particle size -- F.S.S. (Fisher sub-sieve sizer in microns), PA1 3. Surface area -- B.E.T. (gas absorption method of Brunouer, Emmett and Teller in meters.sup.2 per gram).
When the ratio of (F.S.S.).sup.2 /B.E.T. for a UO.sub.2 powder is between 0.025 and 0.40 excellent pellets are produced. A desirable optimum range for the average values for a series of lots for (F.S.S.).sup.2 /B.E.T. is from 0.06 to 0.13, excluding recycled UO.sub.2.